We got an electricity bill a few days ago for nearly £300 ($600). The problem is, the bill was from a company we stopped using a year ago (EDF). We have the year-old "final bill" from EDF, and we've been getting bills from our actual supplier (Green Energy UK) so it's almost certainly an error. Sigh... So I'll be navigating the EDF phone trees on Monday.
When I worked for Headway working with people with brain injuries, this would happen all the time. About once a month, one of my clients would receive a strange bill from an electric company they had not used in years. One company, PowerGen, was particularly egregious, and sent very threatening letters with high frequency to one of my clients. Every month, I would call them, explain the situation, and they would apologize, acknowledge that my client did not owe any money and say it would not happen again. Then, it would happen again the next month.
UPDATE: After speaking with customer service, it appears this bill I received is genuine... it just covers a 5 month period and is a year late. Huh? Bills are fun!